Recently, laminated metal sheet, in which plastic films such as polyester films are laminated on one side or both sides of a thin metal sheet (in particular, a cold rolled steel sheet with the thickness of from 0.13 to 0.38 mm), instead of a tin-plated steel sheet is being used as material for can container. In such a laminated metal sheet, for laminating a plastic film to a metal sheet, there are the method for heat-bonding by applying the self-adhesiveness of the film, and the method for bonding a plastic film with a metal sheet through a bonding agent. In both cases, a metal sheet is heated, cold plastic films are put on the heated metal sheet, and then the metal sheets are pressed together using a pair of laminating rolls. For example, when laminating plastic films on both sides of a metal sheet, the metal sheet is heated to a temperature higher than the softening point of the films through such apparatus as an electric heating apparatus, a high-frequency heating apparatus, a hot blast stove and others, a cold plastic film is put on both sides of the heated metal sheet, and then the cold plastic film and the heated metal sheet are passed through between a pair of laminating rolls having a rubber lining so that the plastic film is bonded to the metal sheet (refer to Japanese Patent Laid Open No. 8335/1994(Heisei 6)).
In the conventional method for production of a laminated metal sheet in which a metal sheet is heated in laminating, there is no problem when laminating is carried out at a low speed of the order of 20 m/min. However, when laminating at a high speed beyond 150 m/min, the percentage of mixed air bubbles (the ratio of air bubbles) caught between the metal sheet and plastic film of the laminated metal sheet obtained reaches 10 to 30% by the area ratio. When the laminated metal sheet in which air bubbles are mixed is deep-drawn to be formed into can containers, flaws are apt to be caused at such portions on the surface of the laminated metal sheet, which causes the deterioration of products. Further, when the lamination is carried out at a high speed beyond 200 m/min, there are cases in which the adhesion strength becomes uneven, which causes the lowering of the strength of adhesion.
It is a technical solution of the present invention to provide a production method of a laminated metal sheet in which the taking-in of air bubbles is low and the unevenness of adhesion is low even when lamination is carried out at a high speed, and to provide an equipment for the production of a laminated metal sheet used for it, cancelling the above-mentioned problems of the conventional methods.